1. Field of the Invention
This invention relates generally to the operation of high pressure mercury arc lamps and, more particularly, to apparatus and method for operation of arc lamps that increases the stability and lifetime of the lamps and, in addition, can provide an indication the impending catastrophic failure of the mercury arc lamp.
2. Discussion of the Related Art
The need for high intensity light sources in various manufacturing processes has led to the increasing use of high pressure mercury arc lamps. Recently, the wafer stepper activity of the semiconductor industry has required the use of high intensity lamps operating for extended periods of time. Because lamp failure can cause a significant decrease in productivity, resulting from down times and repair of apparatus compromised by an arc lamp failure, and because fluctuations in the lamp operating characteristics are detrimental to the quality of the production process, efforts have been made to stabilize the operation of the lamp. Productivity can frequently be improved in the manufacturing process by higher intensity light sources, and high pressure mercury arc lamps with higher power ratings, have been placed in extended use, for example, in the processing of semiconductor wafers. The lamps with higher power ratings have produced additional control problems.
A flow of cooling air has typically been provided to the lamp for stabilization of the temperature in a temperature region where high arc lamp efficiency has been established. The typical manner of calibration of the flow of air has, in the past, been to insert a temperature sensing element in contact with one of the electrodes, to initiate a lamp arc, and to determine experimentally where the fan controlling the flow of air produces the recommended operating temperature of the arc lamp. Once this parameter is established, the temperature measuring device is removed and the optimum characteristic of operation of the cooling fan is considered to be established for the remainder of the life of the bulb. This procedure has two problems. First, it is typical to initiate operation of the lamp simultaneously with the initiation of the flow of cooling air past the lamp. The flow of air thus lengthens the time which is required by the lamp to reach equilibrium operating conditions. In some instances, the arc lamp may never reach the recommended operating conditions. This situation is especially common when an attempt is made to prolong arc lamp life by operating the lamp under reduced temperature or power conditions. Second, this technique does not take into account that fluctuations exist in line voltages, in operating characteristics of the lamp, and in the ambient atmospheric conditions. The flow of air, for example, can frequently be affected by opening of a door to the room containing the arc lamp, by unnoticed obstacles in that air flow paths as well as other similar conditions.
In previous arc lamp installations, control of characteristics such as temperature, voltage and current was performed manually. The manual control of the characteristics of the mercury arc lamp can require the services of the technician and has proven to be an ineffective method to improve performance of the lamp.
When a lamp fails, it is frequently through a catastrophic event, such as an explosion. Not only are the associated manufacturing processing interrupted, but the disturbance resulting from the explosion, can require extensive repair and maintenance. A desirable feature of any apparatus associated with a high pressure mercury arc lamp would be the ability to warn of impending failure.
A need has therefore been felt for a method of control of the arc lamp that does not require the services of a technician, can stabilize the operating conditions of the arc lamp in a optimum manner, and can indicate when a lamp failure is imminent.